1. Field of the Invention
The present invention relates to a method and apparatus for simultaneously measuring a displacement and angular variations, which allows light radiated from a single light source to be placed along a light axis so as to simultaneously measure a distance (displacement) and angular variations that are different physical quantities, so that the displacement and angular variations of the measuring device of a precision measuring apparatus or machining device of a machine tool moved by a stage can be simultaneously measured without an Abbe error.
2. Description of the Related Art
In general, a precision measuring apparatus or machine tool is equipped with a stage to move a measuring device or machining device to a desired location. When the stage is operated, the stage undergoes a rectilinear movement in a driving-axis direction and a rotation movement (yaw or pitch) in two directions perpendicular to the driving-axis direction. The performance of the stage depends on the extent to which the stage moves accurately and rectilinearly without a rotation movement.
Accordingly, an apparatus for measuring a positional variation (displacement) in the driving-axis direction and angular variations in two perpendicular directions is required to accurately operate the stage and improve the performance of the stage. Since both the displacement and the angular variations cannot be simultaneously measured with a single conventional apparatus, various types of measuring apparatuses have been simultaneously used to measure the displacement and the angular variations.
FIG. 1 is a schematic diagram showing a Michelson interferometer for measuring a displacement.
The P-polarized light of light radiated from a light source 101 is reflected by a polarizing beam splitter 102, passes through a quarter wave plate 105 and then is projected on a reference mirror 106. In contrast, the S-polarized light of the light passes through the polarizing beam splitter 102 and a quarter wave plate 103 and is projected on a stage mirror 104 moving along with a stage.
The P-polarized light, which is linear polarized light and was projected on the reference mirror 106 through the quarter wave plate 105 in the form of circular polarized light, is converted into S-polarized light having a perpendicularly converted direction while passing through the quarter wave plate 105 again, passes through the polarizing beam splitter 102 and is projected on a linear polarizer 107. This S-polarized light is converted into linear polarized light having both an S-polarized light component and a P-polarized light component by the linear polarizer 107 placed at an angle of 45°, and is projected on a photo diode 108 to be used as reference light to measure a distance.
The S-polarized light, which is circular polarized light and was projected on the stage mirror 104 through the quarter wave plate 103 in the form of circular polarized light, is reflected by the stage mirror 104, converted into P-polarized light having a perpendicularly converted direction while passing through the quarter wave plate 103 again, and then reflected by the polarizing beam splitter 102. This P-polarized light is converted into linear polarized light having both S-polarized light and P-polarized light while passing through the linear polarizer 107 placed at an angle of 45°, and is projected on the photo diode 108 to be used as light to measure a distance.
Accordingly, a signal processing circuit (not shown) and a computer (not shown) connected to the photo diode 108 analyze the phase difference between the light reflected by the reference mirror 106 and the light reflected by the stage mirror 104, and thus can calculate the distance along which the measuring device of the measuring apparatus or the machining device of the machine tool has moved with the aid of the stage.
Since the laser interferometer measures a displacement in the wavelengths of light, it can precisely measure the displacement at a nanometer level, but cannot measure an angular variation. As occasion demands, both a displacement and an angular variation in a single direction can be simultaneously measured by a modified laser interferometer, but cannot measure a displacement and two angular variations in two directions.
FIG. 2 is a schematic diagram showing the principle of an autocollimator for precisely measuring angular variations.
A part of light radiated from a light source 201 through a pin hole 202 passes through a beam splitter 203, is converted into a collimated beam while passing through a lens 204, and then is projected on a stage mirror 205. The remaining part of the light is reflected by the beam splitter 203 (not shown).
A part of the light which is projected on and reflected by a stage mirror is reflected by the beam splitter 203 and is projected on a position sensitive detector 206, while the remaining part of the light passes through the beam splitter 203 and is projected on the light source 201 (not shown). In this case, the part of the light projected on the position sensitive detector is focused on a single point to allow the incident position thereof to be accurately detected.
Accordingly, a signal processing circuit (not shown) and a computer (not shown) connected to the position sensitive detector 206 analyzes the position of the light projected on the position sensitive detector 206, and thus can calculate the angular variations of the measuring device of the precision measuring apparatus or machining device of the machine tool moved by a stage, in two directions.
The above-described autocollimator is a device for precisely measuring small angular variations. The autocollimator can measure angular variations (yaw and pitch directions), but cannot measure a distance.
Therefore, to measure both a displacement and angular variations, both a laser interferometer and an autocollimator should be employed. However, when a displacement and angular variations are measured by different apparatuses, axes on which two different physical quantities are measured become different and thus an Abbe error occurs, limiting measuring capability. Furthermore, to simultaneously measure a displacement and angular variations by different apparatuses, an additional apparatus is required, and thus costs for measuring the displacement and the angular variations are increased.